1. Field of the Invention
The present invention relates to air intake systems for carburetor-equipped internal combustion engines, and, more particularly, to an air intake system which is designed to automatically control the combustion air temperature by means of a flapper valve that mixes cold raw air with preheated raw air, the flapper position being continuously adjustable by means of a pneumatic membrane actuator.
2. Description of the Prior Art
The prior art in the field of air temperature control systems for carburetor-equipped internal combustion engines includes a number of suggestions for resolving the problem of supplying the internal combustion engine with preheated raw air, at a controlled, optimal temperature, especially during cold start of the engine at low ambient temperatures.
Most air intake systems of this kind feature a warm air intake duct through which preheated raw air passes from a so-called exhaust stove to a duct junction in the cold air intake duct, a pivotable flapper being so arranged in the duct junction that, by progressively opening one intake duct while closing the other, it proportions the flows of cold raw air and preheated raw air, for the establishment of the desired raw air temperature. The angular position of the flapper is set by means of a pneumatic membrane actuator which receives negative pressure through a vacuum line which is connected to the air intake manifold of the internal combustion engine, downstream of the carburetor main throttle.
From U.S. Pat. No. 3,726,512 it is known to arrange in the vacuum line between the intake manifold and the pneumatic actuator a temperature-responsive relief valve which is designed to reduce the negative pressure in the vacuum line, when the raw air temperature reaches a certain level. This known thermostat relief valve features a valve housing which is mounted in the clean air space of the air intake filter, with the two vacuum line connections reaching to the outside of the filter housing.
The valve mechanism of this relief valve includes a moving valve element in the form of a ball which cooperates with a relief port, the movement freedom of the ball being controlled by the curvature of a bimetallic member. In the cold state of the thermostat relief valve, the bimetallic member presses the ball against the relief port, so that the full effect of the intake manifold vacuum is transmitted to the pneumatic membrane actuator. As soon as the temperature of the combustion air flowing through the intake filter has reached a predetermined level, the bimetallic member assumes a curvature in which it allows the ball to move away from the relief port, thereby admitting air into the vacuum line and reducing the negative pressure in that line, until an equilibrium is established between the degree to which the negative pressure is reduced by the partial opening of the relief valve and the angular position of the air flow proportioning flapper. If the temperature of the incoming combustion air is above the desired level, the thermostat relief valve opens sufficiently to completely eliminate the negative pressure in the vacuum line, so that the pneumatic membrane acutator and the flapper assume their rest position in which the warm air intake duct is closed and only raw air of ambient temperature is drawn into the air intake filter.
In U.S. Pat. No. 3,830,210, it is further suggested to arrange in the vacuum line, between the thermostat relief valve and the pneumatic membrane actuator, a temperature-responsive back pressure valve which, for as long as the internal combustion engine is still below its desired operating temperature, prevents the removal of the negative pressure from the pneumatic membrane actuator, thereby maintaining the warm air intake duct open, in spite of the disappearance of the negative pressure downstream of the carburetor, as a result of a change from partial-load operation to full-load operation, for example. A bimetallic member associated with the back pressure valve assures that, as soon as the engine has reached a certain temperature, the action of the back pressure valve is eliminated, so that, under full-load operation, the engine will take in only cold combustion air, for a maximum energy output.
It is further known from the prior art that the thermostat relief valve and the back pressure valve may be arranged inside a common valve housing, in communication with the incoming combustion air in the clean air space of the intake filter, so that the bimetallic members of the two valves respond to the temperature of the combustion air. Because of its simplicity and low cost, this duplex valve has achieved a degree of acceptance in practical automotive applications.
It has now been found, however, that under conditions of rapid changeover from partial-load operation to full-load operation, the internal combustion engine may be subject to malfunction in the form of "hesitation". This condition is particularly prevalent, when, after a time of full-load operation, the engine is operated briefly at partial load, followed by a resumption of full-load operation. The cold air taken in during the initial full-load operation may have cooled off the thermostatic member of the back pressure valve sufficiently to permit closing of the latter, so that the negative pressure created during partial-load operation is maintained in the membrane actuator, long after resumption of the full-load operation. This means that the volumetric efficiency of the engine is lowered, through the intake of preheated air, until the bimetallic member of the back pressure valve responds to that preheated air by inactivating the back pressure valve.
Other temperature-responsive arrangements of the back pressure valve, using the temperature of the engine cooling system, or of the engine exhaust system, for example, have been found to be too complex and costly for mass production and automotive application.